Complex plant interactions in heterogeneous material require the ecological rethinking of sowing density recommendations for bread wheat. A review

Summary

Despite the growing concerns over the damages and unsustainability of conventional agriculture, arable farmers struggle to convert to organic cropping practices. In wheat farming, the lack of cultivars adapted to organic cropping has prompted a search for alternatives to better cope with unpredictability and stress. In that respect, heterogeneous material (HM) has attracted a lot of attention for its good performances and yield stability across years and environments. These benefits are thought to arise from facilitative plant interactions brought forth by intraspecifc diversity, but much remains to be known about the mechanisms at play as well as their interactions with the other elements of the cropping system.
Here, we review the literature on plant interactions within organic bread wheat crops through the successive scopes of (i) heterogeneous material, (ii) plant density, and (iii) their interaction. Our major findings are as follows: (1) optimizing heterogeneous material performance and evolutionary trajectories grossly amounts to tipping the balance between competitive and facilitative plant interactions toward the latter. (2) The stress gradient hypothesis applies to the competition/facilitation balance within HM: The more stressful the conditions, the more facilitation happens. (3) Plant density also affects this balance, and the relationship between net facilitation and plant density in HM follow a humped curve. (4) Therefore, the optimal plant density range for HM should be both narrower and lower than for pure lines, and also harder
to predict. (5) High-tillering, high individual yielding plant types should probably be aimed for in HM, for two reasons: first, they perform better at the lower-than-the-recommendations plant densities at which HM are expected to best express their potential. Second, optimal plant densities of such plant types are more stable across environments, which should increase the probability of falling within the narrower optimal plant density range of HM, particularly in unpredictable and/or stressed environments.